The present disclosure relates generally to a spinal fixation device and, in particular, to a dynamic spinal fixation system and method of use for stabilizing one or more levels of the cervical spine or lumbar spine as well as to a spinal fixation system including attachment portions with reliefs. When non-surgical treatments of spinal injuries, diseases, and trauma fail, anterior spinal surgery is often performed to access the cervical or lumbar vertebrae or intervertebral discs. The anterior spinal surgery that is performed may be an anterior cervical discectomy and fusion (“ACDF”). During an ACDF procedure a bone graft or interbody implant is often used to replace the removed disc and a spinal fixation plate is then attached to adjacent vertebrae to stabilize the spine and foster arthrodesis. Current procedures employ placement of the plate first and the screws to fix that plate to vertebrae second. Most commonly, spinal fixation plates are affixed to the vertebrae using bone screws.
The currently available spinal fixation plates or devices limit visualization of the vertebrae during placement. In addition, currently available spinal fixation devices are difficult to place along the midline. The currently available spinal fixation devices also create an inability to align intervening segments for fixation. Finally, the currently available spinal fixations devices make it difficult to pull the vertebrae up into a more lordotic position when significant kyphosis exists.
Accordingly, the present invention contemplates new and improved spinal fixation systems which overcome the above-referenced problems and others.
Bone fractures are a common occurrence. In many cases, conservative treatment, such as closed reduction and casting, will result in successful stabilization of the injured bone or bones for healing. In other cases, such as more unstable fractures or fractures in bones with limited blood supply, the fracture may require surgical treatment. One common surgical treatment is open reduction internal fixation (ORIF). A wide array of implants can be used during ORIF, including but not limited to: plates, screws, pins, staples, wires, hooks, or combinations thereof. In the case of plates, bone screws are placed through holes or slots in the plate and are used to affix adjacent bone segments, thus stabilizing the fracture while healing takes place. Similar techniques are used for fusing two adjacent bones when stabilization across a joint is required.
Optimal loading and optimal stabilization have not been achieved with existing implants. Implants which are too compliant can result in excessive motion which can result in a delayed union or non-union of the fracture. Implants which are too rigid can result in stress-shielding, bony resorption, and delayed or non-union of the fracture which often results in plate or screw failure. The ideal mechanical environment is achieved through load-sharing. In the case of a fracture, loads transmitted through the bone are shared between the fracture and the plate. Without the plate, all of the applied loads are transmitted through the fracture. With an extremely rigid plate, none of the applied loads are transmitted through the fracture. In this way, the stiffness of the plate largely dictates the biomechanics experienced by the fracture. Ideally, the plate affords a balance between overloading (and excess motion) and underloading (and stress-shielding).
Seminal research in fracture fixation biomechanics has demonstrated that rigid fixation (<2% strain) leads to primary bone formation. Osteoclasts traverse the fracture and osteoblasts form new bone across the fracture site in the wake of the cutting cone. Less rigid fixation which results in 2%-10% strain leads to secondary bone formation. In this case, a callus forms to further stabilize the fracture and fibrochondrocytes mineralize by endochondral ossification forming woven bone. Eventually the fracture site is remodeled to form lamellar bone. When the fixation is so compliant that it allows >10% strain, most fractures go on to form a fibrous non-union. Some designs of fracture fixation plates, intramedullary rods, and external fixation systems are based on these guiding biomechanical principles.
Traditional reconstruction (recon) plates are static and minimize interfragmentary motion by lagging the bone fragments to the rigid plate via screw fixation. The goal is to fix the bone fragments in a neutral position so that healing can occur. An alternative for facilitating compression of the fracture site is dynamic compression plating (DCP) systems. DCP systems utilize tapered screw holes or slots, which facilitate compression of the fracture fragments across the fracture site as bone screws are tightened to the bone.
Locking plates have gained popularity in recent years. They are used to stabilize fractures in lower density bone commonly found near the ends of long bones (in the metaphyses) or in patients with reduced bone density due to metabolic disease (osteoporosis). Although these plates provide improved fixation in lower density bone, the screw-plate interface is rigid (locked) and thus there is limited load-sharing across the fracture site. The amount of load-sharing is thus dependent on the stiffness of the plate.
Cervical and lumbar spinal pathology is common in all industrialized nations. For patients who fail conservative therapy, spinal surgery is an option. The gold standard for cervical radiculopathy is anterior cervical decompression and fusion (ACDF). A common treatment for lumbar radiculopathy is anterior lumbar interbody fusion (ALIF). The implants, general steps, and goals for each procedure are similar. Commonly, the cervical spine is exposed from an anterior approach and a single level or multiple levels of the spine are exposed. A discectomy and decompression are performed using standard techniques. A bone graft or interbody implant is often placed to fill the vacated disc space and to assist in maintaining disc height. A spinal fixation plate is then used to stabilize the spine and to foster arthrodesis. Spinal fixation plates span a single intervertebral disc and are affixed to two adjacent vertebrae for a single level procedure. When multiple discs are involved in the pathology, a corpectomy is often performed in addition to discectomies at each offending level. In the case of two level fixation with a corpectomy, the spinal fixation plate spans two intervertebral discs and affixes to the adjacent vertebrae. In the case of two level fixation without a corpectomy, the spinal fixation plate spans two intervertebral disc and affixes to both the adjacent and intervening vertebrae. Most commonly, spinal fixation plates are affixed to the vertebrae using bone screws.
One skilled in the art recognizes that midline placement of the plate is critical to the success of the ACDF. However, existing techniques and plates are generally limited in that they do not allow the user (surgeon) to visualize the vertebrae during placement.
One skilled in the art further recognizes that dynamic fixation of the spine with a dynamic fixation plate is advantageous for load sharing and fostering interbody fusion. Interbody motion in flexion/extension is particularly advantageous, while anterior/posterior shear motion is disadvantageous.
During surgery and placement of the fixation plate, it is often necessary to adjust the lordotic curve of the spine to a more anatomic shape. Using standard techniques, the fixation plate is often used to “pull” the vertebrae into shape by lagging the vertebrae to the plate using bone screws.
An elastic element is one that deforms when loaded and returns to its original shape (or near original shape) when unloaded. An elastic element will deform when loaded in tension or compression (or bending or torsion). In one special case, elastic deformation may be linearly proportional to load as shown in FIG. 96. In this special case, force (F) causes a deformation (d) that is linearly proportional to the force by a constant (k) and is governed by F=kd. Those skilled in the art will recognize that the relationship between force and deformation does not have to be linear, but could also be non-linear.